1. Field of the Invention
The present invention relates to an air filter used to clean air in a clean room and air for cleaning devices in the fabrication of, for example, semiconductors, liquid crystals, and optoelectronics.
2. Description of Background Art
In processes for the fabrication of silicon wafers and the like in semiconductor plants, contaminants such as particles and metal impurities in a clean room cause defects of devices integrated to the extent of 64M bit DRAMs. These contaminants are therefore removed to the minimum using a HEPA filter or ULPA filter. However, in the case of devices integrated to a degree greater than 64M bit DRAM, ppb level gas contaminants in a clean room decrease the production yield. These gas contaminants are known to be originated from outside air, gas generated from the structural parts of the clean room, and chemicals used in the processes.
To reduce these gas contaminants to a concentration level of the order of ppm or less, a method of removing these contaminants are removed using a chemical filter in the clean room is adopted. The chemical filter removes gas contaminants, for instance, by passing the air in a clean room and devices through a filter of an adsorbent such as activated carbon. The gas contaminants are classified into acids, alkalis, and organic substances. To efficiently remove these gas contaminants, the chemical filter generally contains an appropriately selected adsorbent or reactive material absorbing these gases as a carrier.
For instance, amines are contained in an amount of several tens ppm in air and invade into a clean room when the outside air is introduced into the clean room. Also, it is known that basic gas contaminants such as amines are generated from building materials for a clean room and treating chemicals. If these basic gas contaminants exist in an amount of 10 to 20 ppb in a clean room, these contaminants give rise to abnormality in the shape of a photo resistance (T top phenomenon). In addition the basic gases react with acidic gases and form salts, which make the surfaces of optical parts in process equipment and the surfaces of silicon wafers to become cloudy. For this reason, a chemical filter carrying an acidic substance as a reactive material for absorbing gases is used to remove basic gas contaminants such as amines. If acidic gas contaminants exist in a clean room, on the other hand, these contaminants corrode glass fiber of a dust filter to thereby promote the generation of boron and also cause the corrosion of metals such as ICs.
Chemical filters have been used to remove such chemical substances. These chemical filters are manufactured from a porous adsorbent, with or without addition of a spreader, by processing such a porous adsorbent into the form of an air filter, and can remove chemical substances from a gas to be treated passing therethrough. Many chemical filters are used because they can remove various chemical substances. However, these chemical filters have a limitation to the amount of adsorption because they adsorb chemical substances by physical or chemical adsorption. It is therefore necessary to replace these filters after they are used for a certain period of time. The frequency of replacement is usually once several months to once a year, for example, although the frequency depends on working conditions. The replacement involves cost for purchasing new filters as well as maintenance costs. In addition, replacement generates waste materials.
To avoid such a problem, a method using a rotating regenerative type chemical filter is disclosed in the publication of JP-A-2000-296309. This method employs a rotor type chemical filter, which is rotated to adsorb chemical substances in the process air passing through a treating zone. The adsorbed chemical substances are heated in a regenerating zone to desorb and remove the chemical substances. Then, the filter is cooled in a purge zone. These steps are repeated to regenerate the filter continuously. The use of this method enables the filter to be regenerated many times until the adsorbent is deteriorated, saving the necessity for purchasing new filters and for disposal of used filters.
However, because the rotor continues rotating in this method, the regeneration zone must be always heated, resulting in an increased power consumption. In addition, the air after treating is eventually dried due to moisture adsorption by the adsorbent. In a process for the fabrication of semiconductors, the humidity must be controlled at a given level from the reasons described below.
Namely, the temperature and moisture in a clean room must be kept at a certain range. This is because if the temperature and moisture in the clean room exceed the upper limit of this range, the excessive moisture content caused moisture deposited onto the surfaces of silicon substrates. Such moisture deposited on the surfaces may cause acids, alkalis, and organic substances to be dissolved therein. The excessive moisture content also may oxidize the substrate and the like. If the temperature and moisture in a clean room are less than the above range, particles are adsorbed electrostatically onto the surfaces of the silicon substrates and the like because of the excessively small moisture content.
Therefore, in order to maintain a constant moisture content, the temperature and moisture of the air passing through a treating zone has conventionally been detected using a sensor or the like and the air has been treated with pure water or using a humidifier and the like.
However, this method has problems such as cost involved in the treating, requirement for large equipment, and the like.
Therefore, it is an object of the present invention to provide an inexpensive, compact, and long-life air filter.
In view of this situation, the inventors of the present invention have conducted earnest studies and as a result, found that the running cost of an air filter can be suppressed if a rotor is intermittently rotated by suitably selecting the treating time, regenerating time, and purge time, for example, a xe2x85x9 revolution, once every 6 hours, rather than always rotating the rotor. This fining has led to the completion of the present invention.
Accordingly, the present invention provides an air filter comprising a honeycomb rotor which carries a porous adsorbent and can be rotated in the circumferential direction, a drive means for rotating the honeycomb rotor, a first gas introduction section disposed on one side of the honeycomb rotor and a second gas introduction section disposed on another side of the honeycomb rotor, the first and second gas introduction sections introducing counter-currently flowing two types of gases into different conduits, a first gas exhaust section which is disposed on the other side of the honeycomb rotor and introduces the gas which is introduced from the first gas introduction section and exhausted through the honeycomb rotor, a second gas exhaust section which is disposed on the one side of the honeycomb rotor and introduces the gas which is introduced from the second gas introduction section and exhausted through the honeycomb rotor, a heater disposed at the inlet port of the first gas introduction section, and a drive controller for controlling the drive means so that the honeycomb rotor can rotate intermittently by every part of a regenerative/purge zone facing the first gas introduction section in the filter surface of the honeycomb rotor.
According to the above invention, the time required to operate the heater is shortened because the heater is turned off as soon as the regeneration is finished in the regenerative zone. In addition, because the regenerative zone is used as a purge zone as is after is turning off the heater, the power consumed for rotating the honeycomb rotor can be significantly saved. Also, because a purge operation is carried out using the air in the clean room, the humidity can be maintained constant before and after the purge operation.